Vehicles may be fitted with evaporative emission control systems to reduce the release of fuel vapors to the atmosphere. For example, vaporized hydrocarbons (HCs) from a fuel tank may be stored in a fuel vapor canister packed with an adsorbent which adsorbs and stores the vapors. At a later time, when the engine is in operation, the evaporative emission control system allows the vapors to be purged into the engine intake manifold for use as fuel. However, leaks in the emissions control system can inadvertently allow fuel vapor to escape to the atmosphere. Thus, various approaches are used to identify such leaks.
One example leak detection method is described in U.S. Pat. No. 5,575,265. Here, a valve containing a reference orifice is utilized to determine the pressure or vacuum of the fuel system in the presence of a small leak, such as the EPA standard 0.5 mm. The pressure or vacuum determined while the system is closed can then be compared to that obtained with the reference orifice in the system.
However, the inventors herein have recognized a problem with the above approach. Even when a known reference leak is used in the system, vacuum or pressure can be affected by the movement of the various valves that couple the known leak into the system. Furthermore, even when the known leak can be used in the system, the variation in the vacuum or pressure generated relative to atmosphere can may not be sufficiently equalized with atmosphere between obtaining the reference and test signals, thus masking differences in pressure or vacuum between the two and increasing the chance of an erroneous determination.
Thus, in one example, the above issue may be addressed by a system comprising a vapor line coupling a tank to a hydrocarbon filter, a vent line between the hydrocarbon filter and atmosphere, and a three-way valve in the vent line between an ambient filter and the hydrocarbon filter. The valve has a neutral spool position coupling the hydrocarbon filter with atmosphere, a reference spool position coupling the hydrocarbon filter with atmosphere, and a test spool position opposite of the reference position blocking atmosphere from the hydrocarbon filter.
In this manner, the three ports needed to operate the fuel recovery system in purge, storage, reference, and test modes can all be arranged on one valve. Furthermore, the arrangement of the ports allows for a simplified testing strategy that equalizes the pressure in the system back to atmospheric between each reference and testing activity. Therefore, control issues and background noise introduced by transitioning between the various positions can be minimized.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.